A. Field of Invention
This application pertains to an ultrasonic scanning probe having a tuning fork-type oscillator supporting an ultrasonic generator, as well as a control circuit for controlling the lateral movement of the oscillator.
B. Description of the Prior Art
Ophthalmic ultrasound is a diagnostic medical imaging technology that utilizes high frequency sound waves to create cross-section B-scan images and time-amplitude A-scan images of the globe and orbit of the eye. As the sound waves strike intraocular structures, they are reflected back to the probe and converted into an electric signal (FIG. 1.1). Ultrasound is a safe, noninvasive diagnostic tool that provides instant feedback for the evaluation of various ophthalmic disorders. Ophthalmic ultrasound instruments use pulse-echo system, which consists of a series of emitted pulses of sound, each followed by a brief pause (microseconds) for the receiving of echoes and processing to the display screen. A short acoustic pulse is generated mechanically by a piezoelectric crystal, which acts as a transducer to convert electric energy into ultrasound. At every acoustic interface, some of the echoes are reflected back to the transducer, indicating a change in tissue density. The echoes returned to the probe are converted back into an electrical signal and processed as ultrasound images. Currently, ophthalmic ultrasound machines use frequencies in the range of 8 to 80 MHz, compared with 2 to 6 MHz typically used in other fields of diagnostic ultrasound.
A-scan, B-scan and ultrasound biomicroscopy are the most commonly used ultrasound instrumentation techniques. A-scan is a one-dimensional display of echo strength over time. The vertical spikes correspond to echo intensity and are shown the horizontal axis as function of time (FIG. 1.1). It uses frequency of 10 to 12 MHz and is mainly used for axial eye length measurements. A-scan sonography is particularly useful in ophthalmology as a biometric tool where the axial measurement of the globe (the region in the eye from anterior cornea to retinal surface) is a prime data element in the calculations for determining the appropriate power of an intraocular lens implant for cataract surgery.
B-scan is a two dimensional display of echoes using horizontal and vertical orientations to show shape and location (FIG. 1.2). It is an important tool for the clinical assessment of various ocular and orbital diseases. In situations in which normal examination is not possible, such as lid problems, corneal opacities dense cataracts, or vitreous opacities, diagnostic B-scan ultrasound can accurately image intraocular structures and give valuable information on the status of the lens, retina, and other parts. Ultrasound biomicroscopy is an ultrasound instrument that uses frequencies from 35 to 80 MHz for the acoustic evaluation of anterior segment of the eye. Interpretation of the images for all the above ultrasound techniques is based on knowledge of both the normal and abnormal ocular anatomy, and an understanding of the physical principles of ultrasound.
In a pulse-echo-imaging system, one can either scan the transducer in a freehand form and detect the position of the transducer, or control the motion of the transducer. The former was once a popular technique; however the latter is the current technology. A single transducer is scanned mechanically at intervals across the eye that is elliptically shaped. At each controlled mechanical stopping point, sound is sent across and echoes are received. The bright dots in each trace on the display indicate the front and back wall echoes of the eye. By scanning across the eye, multiple lines produce the image of the eye on the display. In order to maintain fidelity of the 2-dimensional images of the near and far structures in the eye, the ultrasonic beams have to diverge from virtually the same point. This means that the image has to be generated by a single beam originating from the same point, being deflected in different angles to build a sector image. This is usually achieved by a single transducer or array sending a single beam that is stepwise rotated, either mechanically or electronically. The subsequent lines of the image are then formed by a slight angular rotation, making the beam sweep across a sector as shown in FIG. 1.3.
Mechanical scanning is currently the industry standard in ophthalmic applications like A-Scan and B-scan as it is mostly useful in the 15 to 20 MHz range. Despite the disadvantages associated with moving parts of the scanner like wear and tear, vibration, and so on, the amounts of acoustic noise these scanners produce, compared to phased-array probes, are much smaller. Two types of mechanical scanning systems are commonly used. In a linear mechanical scanner, the transducer is driven back and forth inside an enclosed water bath rather than air at the end of a handheld probe (FIG. 1.4) as the transducer-air interface, acting to acutely refract the sound wave, would result in total internal reflection of the sound wave, and therefore no transmission of the sound would take place. The cable between the probe and the machine contains a coaxial cable, carrying signals to and from the transducer, and leads carrying the drive current to the electric motor and the signal from the position sensor.
Another type of mechanical scanning probe is the sector scanner. The probe contains similar features to the linear scanner but it is designed to produce a rocking motion. The probe is made compact by having the necessary drive components built into and closely around the rocking transducer assembly as shown in FIG. 1.5.
In most non-ophthalmic instruments, linear and phased array transducers have replaced mechanically scanned transducers described above. These arrays can electronically steer and focus the ultrasound beam at various depths and can provide higher refresh rates. However, the cost and technical complications with building arrays increases with frequency. Hence, the current industry trend is to either select expensive arrays to maintain the quality of ultrasonic images or to select the simpler mechanical scanner and compromise the quality, especially for higher frequencies. There is no optimum solution that incorporates both the relative technical simplicity and low building cost of a mechanical scanner and the high performance capability of phased arrays in one device.